Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses, added emphasis has been placed on efficiencies associated with well completions and maintenance over the life of the well. Over the years, ever increasing well depths and sophisticated architecture have made reductions in time and effort spent in completions and maintenance operations of even greater focus.
Well stimulating applications which include perforating and fracturing of a cased well during completions constitute one such area were significant amounts of time and effort are spent. This is particularly true where increases in well depths and sophisticated architecture are encountered. Once the casing hardware is cemented in place, stimulating applications generally take place in a zone by zone fashion. For example, a terminal end of the well may be perforated and fractured followed by setting of a plug immediately uphole thereof. Thus, with the lowermost zone initially stimulated, the zone above the plug may now also be stimulated by way of repeating the perforating and fracturing applications. This time consuming sequence of plug setting, perforating and then fracturing is repeated for each zone. That is, likely 15-20 zones or more of a given well may be stimulated in this manner. Further, for any given zone, each step of plug setting, perforating and fracturing requires its own dedicated application trip into the well via wireline from surface or other appropriate conveyance.
All in all, where stimulating operations are involved, the operator is likely faced with days' worth of time dedicated to the task. In today's dollars this may translate into several hundred thousand dollars of lost time. Once more, footspace at the surface of the oilfield adjacent the well is taken up by simultaneously competing types of equipment. For example, since each zone requires separate dedicated applications of plugging, perforating and fracturing, all such equipment must remain at the oilfield surface throughout stimulation operations. Thus, so as to be available for later use, frac trucks are left running in place after use in one zone so as to be available for use in the next zone. In fact, this particular inefficiency is often exacerbated where a continuously running but intermittently utilized frac truck breaks down due to repetitive cycles of pumping and powering down to allow for plugging and perforating.
Ultimately, once each zone has been stimulated, the well is left with twenty or so isolated zones. Thus, a milling application may ensue where a milling tool is dropped through the well which mills out all of the plugs. As such, flow through the central bore of the well may be restored. Unlike the previous steps, at least the milling may take place through each zone with only one trip into the well with the milling tool.
Efforts have been undertaken to reduce the overall time and number of trips into the well that result from the zone by zone and stepped nature of stimulation operations. For example, the casing at each zone may be outfitted with a shifting sleeve that also includes a ball seat such that the sleeve may be opened and the wellbore exposed to the surrounding formation. That is, rather than separately introducing perforating and fracturing equipment into the well during separate dedicated trips to each zone, ball actuation may be used to open the sleeves one by one for targeted stimulation. That is to say, a ball of appropriate size may be dropped into the well, eventually finding the seat and sleeve of corresponding size and pressurizably opening that sleeve. The ball and seat may then serve the isolation function and the opened sleeve may obviate the need for perforating. Therefore, stimulation of the zone may take place with only the introduction of fracturing equipment.
In theory the above ball drop technique may save a significant amount of time and trips into the well for sake of stimulation. Unfortunately, such a system renders a host of challenges to the rest of well operations. That is to say, as noted below, applications before and after stimulation are likely to be adversely affected by the use of conventional ball-drop and sleeve shifting hardware.
Conventional ball-drop and sleeve shifting hardware requires fairly complex architecture that is incorporated into the casing and present from the outset of completions. This sophisticated architecture includes the noted sleeve which is likely to present a significant restriction into the main bore of the well. Further, complex mechanical parts such as springs, pressure support mechanisms, ratchets and other features of the ball seat are also likely to protrude into the main bore. Thus, as a practical matter, in spite of the potential time saving benefits, operators are likely to forego ball-drop sleeve shifting stimulation techniques.